Method and apparatus for echo cancellation in digital communications using an echo cancellation reference signal

ABSTRACT

A novel echo cancellation reference (“ECR”) training signal is inserted into the current ATSC 8 VSB data stream to achieve improved echo rejection while maintaining compatibility with the ATSC 8 VSB digital ATV standard. The novel ECR training signal is also suitable for other multipath or dispersive digital communication channels. A pre-equalization subsystem is included in the digital ATV system as a front end to the VSB receiver, and is trained with preferably the novel ECR training signal. One type of pre-equalization subsystem provides a fully ATSC compliant signal at its output, and therefore is particularly useful with standard VSB receivers. Another type of pre-equalization system provides a signal at its output which is ATSC compliant in some respects but which retains the ECR signal, and therefore is particularly useful for signal relays, analysis purposes, and other applications in which the residual channel information is important.

BACKGROUND OF THE INVENTION

[0001] Communication engineering continually must deal with the problemof restoring a signal that has been altered by the communication pathover which the signal was transmitted. Signal restoration often can beachieved if the communication path is filly characterized, at least asto those parameters that contribute to the signal alteration. Thus, afrequently essential component of the signal restoration problem is thatof identifying the characteristics of the communication path or channel.

[0002] One approach to the channel identification problem is to transmita cancellation reference signal having a known characteristic, over thechannel, and to receive the transmitted signal after it has passedthrough the channel. The originally transmitted signal is compared withthe received signal, and a model of the channel characteristics isdeveloped based on the comparison. One type of cancellation referencesignal useful for correcting echo interference is known as an echocancellation reference (“ECR”) signal. Echo interference, which is alsoknown as multipath or dispersive interference, affects analog anddigital communications signals. An example of a system and ECR forimproved echo cancellation in analog television receivers is describedin U.S. Pat. No. 5,121,211, issued Jun. 9, 1992 to David Koo. Anothersystem and architecture for echo cancellation suitable for televisionreceives is described in U.S. Pat. No. 5,278,872, issued Jan. 11, 1994to Craig B. Greenberg; and in U.S. Pat. No. 5,396,299, issued Mar. 7,1995 to Craig B. Greenberg. An ECR also has been specified by theAdvanced Television Systems Committee (“ATSC”) of the United States, andis described in the following document: Advanced Television SystemsCommittee, Standard A/49: Ghost Canceling Reference Signal for NTSC,Approved Aug. 14, 1992 and Modified May 13, 1993.

[0003] A digital Advanced Television System (“ATV”) has now beenspecified for the United States. The characteristics of the digital ATVare documented in various standards of the Advanced Television SystemsCommittee (“ATSC”) and are available from the ATSC, 1750 K Street N.W.,Suite 1200 Washington, D.C. 20006. Basically, the Digital TelevisionStandard describes a system designed to transmit high quality video andaudio and ancillary data over a single 6 MHz channel. The system candeliver reliably about 19 Mbps of throughput in a 6 MHz terrestrialbroadcasting channel and about 38 Mbps of throughput in a 6 MHz cabletelevision channel. Although the RF/Transmission subsystems described inthe Digital Television Standard are designed specifically forterrestrial and cable applications, the objective is that the video,audio, and service multiplex/transport subsystems be useful in otherapplications. Further general information about the Digital TelevisionStandard is presented in the following publication: Advanced TelevisionSystems Committee, Standard A/54: Guide to the Use of the ATSC DigitalTelevision Standard, Oct. 4, 1995.

[0004] One component of the digital ATV is known as “RF/Transmission,”which refers to channel coding and modulation. The channel coder takesthe data bit stream and adds additional information that can be used bythe receiver to reconstruct the data from the received signal, which,due to transmission impairments, may not accurately represent thetransmitted signal. The modulation (or physical layer) uses the digitaldata stream information to modulate the transmitted signal. One mode ofthe modulation subsystem is the terrestrial broadcast mode, also knownas “8 VSB,” which uses vestigial sideband modulation with 8 discreteamplitude levels.

[0005] As more fully described in the above-referenced Standard A/54,the VSB signal contains features, which allow design of receivers thatperform the functions of acquiring and locking to the transmittedsignal. The equalization of the signal for channel frequency responseand echoes uses a training signal, and is facilitated by the inclusionof specific features in the Data Field Sync. Utilization of thesefeatures is made more reliable by the provision of means to firstacquire and synchronize to the VSB signal, particularly by the SegmentSync. The Data Field Sync then can be used both to identify itself andto further perform equalization of linear transmission distortions. TheVSB signal may also be equalized by databased or blind equalizationmethods, which do not use the Data Field Sync. Blind equalizationmethods are more fully described in the above-referenced Standard A/54.

[0006] A standard VSB receiver is shown in FIG. 1, and includes a tuner1, an IF filter and synchronous detector 3, synchronization and timingcircuits 5, an NTSC rejection filter 7, an equalizer 9 for equalizationof the signal for channel frequency response and echoes, a phase tracker11, a trellis decoder 13, a data de-interleaver 15, a Reed-Solomondecoder 17, and a data de-ramdomizer 19. Standard VSB receivers also areavailable from various manufacturers, including the IEEE 1394/Device BayATSC DTV receiver reference design available from PhilipsSemiconductors, Philips Electronics N.V., 811 East Arques Avenue,Sunnyvale, Calif., 94088-3409 (incorporates a Philips type TDA8960demodulator/decoder single chip component).

[0007] Despite significant advancements in today's digitaltelecommunications, echo interference remains one of the most damagingdistortions within the communication links. Strong echo interference cancollapse digital communication links altogether. The collapse of thedigital communication link is due to an intermediate distortion scenariocalled inter-symbol interference (“ISI”), which results from theinteractions between the echo conditions and the transmitted dataitself.

[0008] Various proposals and methods are know or are in practice todayto reduce the impact of echo interference which use equalizers whoseweights are primarily from adaptation and mainly based on statisticaldata information obtained from the multipath or dispersive communicationchannels. Some of these proposals and methods are described in thefollowing article: David Koo, “Ghost Cancellation with ITU System-Cstandard Ghost Cancellation Reference Signal,” Journal of the Society ofMotion Picture and Television Engineers, June 1995, PP. 370-376.Nonetheless, the current ATSC 8 VSB digital ATV system remains heavilysusceptible to echo interference, even though it is not noise limited.It is therefore desirable to improve the performance of the ATSC 8 VSBdigital ATV system as well as other similar digital communicationssystems in the presence of echoes while maintaining their advantage in awhite noise environment.

BRIEF SUMMARY OF THE INVENTION

[0009] One embodiment of the invention is an equalizer comprising anextraction circuit for extracting a copy of an echo cancellationreference signal from a digital data stream transmitted over atransmission path; a microprocessor coupled to the extraction circuitfor calculating filter coefficients from the extracted copy of the echocancellation reference signal; and a filter receiving the digital datastream at an input thereof and coupled to the microprocessor forreceiving the filter coefficients and substantially canceling echointerference from the digital data stream using the filter coefficients.The echo cancellation reference signal is non-cyclic, has asubstantially flat frequency response within the bandwidth of thetransmission path, has a large plurality of amplitude peaks over a timeinterval, and has proportionally shorter tails relative to the largeplurality of amplitude peaks.

[0010] Another embodiment of the invention is an apparatus forsubstantially eliminating echoes occurring during transmission of adigital data stream comprising a plurality of field syncs over atransmission path, a plurality of segments of symbol spaces beingbetween successive ones of the field syncs, and a plurality of echocancellation reference signals between the field snycs. The apparatuscomprises means for receiving the digital data stream aftertransmission; means for extracting copies of the echo cancellationreference signals from the received digital data stream; means forcalculating filter coefficients from the extracted copies of the echocancellation reference signals; and means for substantially cancelingecho interference from the received digital data stream using the filtercoefficients. The echo cancellation reference signals are non-cyclic,have a substantially flat frequency response within the bandwidth of thetransmission path, have a large plurality of amplitude peaks over a timeinterval, and have proportionally shorter tails relative to the largeplurality of amplitude peaks.

[0011] A further embodiment of the invention is an apparatus formodifying an ATSC VSB data stream prior to transmission to a receiver tosupport an enhanced echo cancellation capability in the receiver, theATSC VSB data stream comprising a plurality of field syncs and aplurality of segments of symbol spaces between successive ones of thefield syncs, the apparatus comprising means for inserting a plurality ofecho cancellation reference signals between the field snycs, the echocancellation reference signals being of a class of signals substantiallydefined by${f(t)} = {{\frac{1}{2\pi}{\int_{0}^{\Omega}{\left\lbrack {{A\quad {\cos \left( {b\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {b\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}} + {\frac{1}{2\pi}{\int_{- \Omega}^{0}{\left\lbrack {{A\quad {\cos \left( {{- b}\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {{- b}\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}}}$

[0012] wherein A, b and Ω are real numbers.

[0013] Yet another embodiment of the invention is a method forsubstantially eliminating echoes occurring during transmission of adigital data stream comprising a plurality of field syncs over atransmission path, a plurality of segments of symbol spaces beingbetween successive ones of the field syncs, and a plurality of echocancellation reference signals between the field snycs. The methodcomprises receiving the digital data stream after transmission;extracting copies of the echo cancellation reference signals from thereceived digital data stream; calculating filter coefficients from theextracted copies of the echo cancellation reference signals; andsubstantially canceling echo interference from the received digital datastream using the filter coefficients. The echo cancellation referencesignals are non-cyclic, have a substantially flat frequency responsewithin the bandwidth of the transmission path, have a large plurality ofamplitude peaks over a time interval, and have proportionally shortertails relative to the large plurality of amplitude peaks.

[0014] A further embodiment of the invention is a method for modifyingan ATSC VSB data stream prior to transmission to a receiver to supportan enhanced echo cancellation capability in the receiver, the ATSC VSBdata stream comprising a plurality of field syncs and a plurality ofsegments of symbol spaces between successive ones of the field syncs,the method comprising inserting a plurality of echo cancellationreference signals between the field snycs, the echo cancellationreference signals being of a class of signals substantially defined by${f(t)} = {{\frac{1}{2\pi}{\int_{0}^{\Omega}{\left\lbrack {{A\quad {\cos \left( {b\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {b\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}} + {\frac{1}{2\pi}{\int_{- \Omega}^{0}{\left\lbrack {{A\quad {\cos \left( {{- b}\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {{- b}\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}}}$

[0015] wherein A, b and Ω are real numbers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0016]FIG. 1 is a block diagram of a conventional VSB receiver.

[0017]FIG. 2 is a block diagram of a digital ATV system thatincorporates a pre-equalization subsystem that removes the ECR signalafter equalization.

[0018]FIG. 3 is a schematic diagram of an ATSC data stream for thedigital ATV system of FIG. 2 which contains an echo cancellationreference signal at various locations.

[0019]FIG. 4 is a block diagram of a digital ATV system thatincorporates a pre-equalization subsystem that does not remove the ECRsignal after equalization.

[0020]FIG. 5 is a schematic diagram of an ATSC data stream for thedigital ATV system of FIG. 4 which contains an echo cancellationreference signal at various locations.

[0021]FIG. 6 is a block diagram of an equalizer suitable for the digitalATV systems of FIGS. 2 and 4.

[0022]FIG. 7 is an illustrative waveform of the signal function of timefrom a class of echo cancellation reference signals suitable for theATSC data streams of FIGS. 3 and 5.

[0023]FIG. 8 is an illustrative waveform of a flat spectrum plot from aclass of echo cancellation reference signals suitable for the ATSC datastreams of FIGS. 3 and 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] A novel echo cancellation reference (“ECR”) training signal isinserted into the current ATSC 8 VSB data stream to achieve improvedecho rejection while maintaining compatibility with the ATSC 8 VSBdigital ATV standard. The novel ECR training signal is also suitable forother multipath or dispersive digital communication channels. Apre-equalization subsystem is included in the digital ATV system as afront end to the VSB receiver, and is trained with preferably the novelECR training signal. One type of pre-equalization subsystem provides afully ATSC compliant signal at its output, and therefore is particularlyuseful with standard VSB receivers. Another type of pre-equalizationsystem provides a signal at its output which is ATSC compliant in somerespects but which retains the ECR signal, and therefore is particularlyuseful for signal relays, analysis purposes, and other applications inwhich the residual channel information is important. The result isimproved performance of the digital ATV system in the presence of echoeswhile maintaining its advantage in a white noise environment. Theimproved digital ATV system is expected to be able to cope with echoesin the range of ±80 μs and at −10 dBc.

[0025] An example of a digital ATV system 20 which incorporates apre-equalization subsystem that produces a fully ATSC compliant signalat its output is shown in FIG. 2. The improved performance of thedigital ATV system 20 is achieved at the sacrifice of some increase inthe transmission bit rate from 10.76 Msamples/s to 10.98 Msamples/s. Thedigital ATV system 20 includes a standard VSB receiver 40 such as, forexample, the VSB receiver shown in FIG. 1. In addition, apre-equalization subsystem 30 that a forefront equalizer 32 is placed infront of the VSB receiver 40. The ATSC data stream containing a suitableECR signal is input to the forefront equalizer 32 instead of the VSBreceiver 40. The forefront equalizer 32 preferably has a relativelylarge time span to completely cover the full echo range. Variousadaptation/initialization schemes are useful for training the equalizer32, including, for example, such well-known schemes as correlation andprecision channel model estimation.

[0026] The forefront equalizer 32 preferably is trained on the novel ECRsignal so that the output of the forefront equalizer 32 is a near cleansignal from which the echo is substantially removed. The added ECRsignal is stripped out of the data stream in a temporary memoryfollowing the forefront equalizer 32 using well-known techniques so thatthe resulting signal becomes an ATSC compliant signal. FIG. 2 shows aFIFO register 34 for this purpose, although many other types of memoryare suitable. This resulting signal is supplied at the output of theFIFO register 34 and is passed to the standard VSB receiver 40. It willbe appreciated that the forefront equalizer 32 and the FIFO register 34may be implemented in discrete integrated circuit chips or as anintegrated solution in a single chip.

[0027] The modification to the ATSC data stream for the digital ATVsystem 20 is shown in the data stream 50 of FIG. 3. In the very samebase-band data symbol stream of the current ATSC 8 VSB standard, acombination of periods of blank symbol spaces and the novel ECR signalis inserted between the field syncs, e.g. field syncs 51 and 54. Thecombination of the periods of blank symbol spaces and the ECR signal isfor convenience referred to as a Macro ECR Assembly (“MEA”). Blanksymbol spaces are used in the MEA to prevent contamination of the ECRdue to post and pre echoes. In the representation of an ATSC data streamshown in FIG. 3, the MEA signals, e.g. MEA signals 52 and 55,illustratively are inserted into locations immediately after the fieldsyncs. Different locations should be attempted in practice, sinceparticular locations may provide benefits under certain conditions,especially in the case of higher temporal rate coverage for dealing withfaster dynamically changing echoes. Advantageously, the insertion of theMEA signals in the current ATSC VSB data stream does not reduce the 312segments available between the field syncs.

[0028] The ECR within the MEA preferably is a high performance echocancellation reference signal that is a mathematically formulated highenergy and high performance signal with a white spectrum. The ECR alsoshould be specifically tailored for the anticipated digital datatransmission conditions, and to receive the minimum of unwantedinterference in the channel characterization process in comparison tothe known signals which are applied as echo reduction references today.Examples of known signals which are applied as echo reduction referencestoday, include pseudo-random sequences, Turner sequences, and windowedsin(x)/(x). Examples of unwanted interference includes contamination andcorrelations from noises (not restricted to Additive White GaussianNoise (“AWGN”) only), self echo-induced data symbols, co-channelinterference and their beating products, adjacent channel signals andtheir beating products, and self in-channel intermodulation products.Generally speaking, a proper MEA arrangement for the whole system is tosupport, enhance and guarantee the required performance of the ECRmentioned above.

[0029] The MEA overhead is estimated to be at about 2% of the wholesystem's resources. This estimated MEA overhead is expected to beadequate for the range of echo delays and their temporal behavior intoday's terrestrial data communication and broadcasting conditions basedon currently available echo field testing statistics, such as thoseavailable from the echo testing reports of the National Association ofBroadcasters. The estimated MEA overhead is subject to change as newecho field testing results are available.

[0030] The echo cancellation/reduction range is estimated to be aboutfrom −80 μs to +82 μs, which is based on the currently available echofield testing statistics. These echo field testing statistics show thatit is quite easy to find relatively long echoes in our cities, such as+51 μs to +53 μs echoes with about 30% in voltage strength in the cityof Sydney, Australia; and +61 μs to +62 μs echoes also with 30% voltagestrength in the city of New York City, USA. Therefore, echoes of +70 μsto +80 μs with large amplitudes are expected to be present in New YorkCity, and similar results are expected to be found in other cities aswell. It is also believed that if long lagging echoes, such as +60 μs to+80 μs, exist with large amplitudes, leading echoes with the same timespan may well be occurring because receiver antennas are not verycarefully positioned by the average consumer. This means that leadingechoes from −61 μs to even longer leading position in time may verylikely exist in large cities.

[0031] Based on the above mentioned tests and analysis, a goodterrestrial digital communication/broadcast standard and its hardwarereceiver system should be able to reject the echoes from leading −80 μsto lagging +80 μs, and even a little longer in order to offer an “alllocation” reliable receptions in bad echo environments and conditions.The digital ATV system 20 system of FIG. 2 is expected to achieve echorejection within the range of −80 μs to +82 μs with large amplitudes,with the MEA overhead being limited to about 2% of the whole systemsymbol stream of the current ATSC standard. Advantageously, the currentATSC standard is not changed with respect to its original payload datarate or and current data arrangements. This will maintain decoding chipcompatibility, and will lead to a relatively quicker hardwarerealization and a relatively simpler and quicker system testing byavoiding any need to retest the performance of the new arrangements ofall the symbols, bits, interleavings, error correction codes, pay loads,and so forth.

[0032] To realize the digital ATV system 20 of FIG. 2 without changingdata payload and any other bit configurations, a feasible transmissionsolution is to shorten the symbol time duration by about 2%. As shown inFIG. 3, the 832 symbols in each segment of the data stream 50 areshorten to about 76 μs. As a result, the base-band bandwidth isincreased by about 2% correspondingly, from the original bandwidth of5.38 MHz to a modified bandwidth of 5.49 MHz. However, the bandwidthincrease in this scheme is so small that this new bandwidth is supportedby all of the RF, IF, base-band and SAW filters of the current receiverfront-ends. For example, the new bandwidth is supported by the tuner 1,the IF filter and synchronous detector 3, and the NTSC rejection filter7 of the conventional VSB receiver of FIG. 1. If desired, the parametersfor the original Square-Root Raised Cosine (SQRC) Filter standard (with11.5% roll-off) could be changed by a very small amount. As long as theSQRC remains mathematically the characteristics of an SQRC, the datarecovery performance will not change.

[0033] The MEA signals 52 and 55 are removed from the substantially echofree data stream at the output of the forefront equalizer 32 by the FIFO34 before the data stream is supplied to the VSB receiver 40. This isshown in FIG. 3 wherein a substantially echo free ATSC compliant datastream 57 retains the field syncs 51 and 54 as well as the 312 segmentsin the areas 53 and 56, but does not contain the MEA signals 52 and 55.After re-clocking to restore the 832 symbols in each segment to 77 μ, asubstantially echo free ATSC data stream 57 which does not have any ECRsor MEAs and fully conforms to the current ATSC 8 VSB standard isavailable at the output of the pre-equalization subsystem 30, and issuitable for decoding by currently existing ATSC 8 VSB chips andsystems.

[0034] With an MEA overhead of about 2%, the ECR insertion rate is at 41ECRs per second, which is coherent with the Field Sync (FS) of thecurrent ATSC 8 VSB standard. Based on this ECR insertion rate, Nyquistsampling theory says the rate of possible fastest dynamic echoes thatcan be cancelled or rejected is at about 20 Hz. This 20 Hz dynamic echorejection rate is expected to cover many, and likely most, of thereception conditions due to the dynamic echo pattern change. However,when, with statistical facts, a higher dynamic echo rejection rate isdefinitely needed, a double rated ECR and MEA may be inserted.Illustratively, two MEA signals (not shown) may be inserted about 156segments away from one another and between two field syncs. This bringsthe fast dynamic echo rejection rate up to 41 Hz, albeit at the penaltyof raising the MEA overhead to about 4%. While this result is graduallybecoming contrary to one of the major advantages of using the currentATSC 8 VSB system, namely the high data payload with very small overhead(high efficiency) in comparison with many other systems, fast dynamicecho rejection is nonetheless available if required.

[0035] A significant advantage of the digital ATV system thatincorporates a pre-equalization subsystem is the subsystem'scompatibility with existing systems and chips. In other words, manycurrent existing ATSC 8 VSB systems from many manufacturers can bereused. One possible implementation for supporting chip reuse is thatthe pre-equalization subsystem 30 (FIG. 2) can be designed as anindependent and self-contained chip or module for being placed directlyin front of many current receiver chips. This is possible because theforefront equalizer 32 is estimated to remove better than 95% —perhapsas much as 98% or more—of the echoes within the incoming data streams,assuming it is under normal to heavy echoing reception conditions withreasonable incoming SNR around 27 to 28 dB. After the echo removalprocess, the ATSC data stream becomes sufficiently echo free and easyfor processing and decoding.

[0036] An example of a digital ATV system 60 that incorporates a type ofpre-equalization subsystem that does not remove the ECRs and MEAs isshown in FIG. 4. The improved performance of the digital ATV system 60is achieved at the expense of a slight decrease in data payload. Thedigital ATV system 60 includes a modified VSB receiver 80. Apre-equalization subsystem 70 having a forefront equalizer 72 is placedin front of the VSB receiver 80. The ATSC data stream is input to theforefront equalizer 72 instead of the VSB receiver 80. The forefrontequalizer 72 is generally similar to the forefront equalizer 32 (FIG. 2)and is trained in generally the same way on preferably the novel type ofECR signal so that the output of the forefront equalizer 72 is a nearclean signal from which the echo is substantially removed. It will beappreciated that the forefront equalizer 72 may be implemented indiscrete integrated circuit chips or as an integrated solution in asingle chip.

[0037] Unlike the pre-equalization subsystem 30 shown in FIG. 2, thepre-equalization subsystem 70 of FIG. 4 does not include a FIFOregister. The whole stream of signal and data is processed withoutremoval of the ECRs and MEAs from the data stream so that the data maybe directly decoded. This is possible because in some applications, theresidual channel information is still carried by the inserted ECR and itis very useful information for signal relays, analysis purposes, and soforth.

[0038] The modification to the ATSC data stream for the digital ATVsystem 60 is shown in the data stream 90 of FIG. 5. A combination ofperiods of blank symbol spaces and the novel ECR signal—the MEA—isinserted into certain locations of the data symbol stream, as is so forthe data stream 50 shown in FIG. 3. However, the MEA signals areinserted into locations within each of the 312 segment areas. Forexample, FIG. 5 shows two 312 segment areas 93 and 96 which containrespective MEA signals 92 and 95 in positions just after the respectivefield syncs 91 and 94.

[0039] The MEA overhead is estimated to be at about 2% of the wholesystem's resources, as is so for the data stream 50 shown in FIG. 3. Theecho cancellation/reduction range is estimated to be at least from −80μs to +82 μs, as is so for the digital ATV system 20 shown in FIG. 2.However, unlike the modification to the ATSC data stream 50 shown inFIG. 3, the modification to the ATSC data stream shown in FIG. 5 doesnot involve changing any of the current data framing structures in theATSC data stream. In particular, the symbol duration is unchanged. Thismeans the whole bandwidth of the data signal is maintained, albeit atthe expense of a 2% reduction in the data payload. Instead of theoriginal 19.28 Mbit/s of the current ATSC 8 VSB standard, the datapayload is reduced to 18.90 Mbit/s. It is believed that this slightlylower data payload can easily be absorbed by suitable programmanagement. Advantageously, the transmission bandwidth and all of thefilters in the VSB receiver 70, from the RF and IF filters to thebase-band filters, are absolutely not affected, and there is no reasonfor the VSB receiver to have any parasitic problems or even be modifiedslightly for the system's bandwidth arrangement.

[0040] However, the VSB receiver 70 does contain some basicmodifications relative to the standard VSB receiver as follows. First,the VSB receiver 70 is capable of identifying and skipping the MEAsignals in the data stream 90. Second, the VSB receiver is capable ofhandling any break up in the convolution codes so that the twelvesequential switching sites operate properly. Third, the VSB receiver 70is capable of picking up data in the data stream and linking it toprevious states. Due to the substitution of the MEA signals for data,certain picture data is likely to be missing so that the data andpossibly the convolutional codes become discontinuous. Both thetransmitter and receiver should make the convolutional codes continuousfrom previous data streams to new data streams. Fourth, the VSB receiver70 has a modified data interleaver that is able to maintain order whenconvolutional codes are discontinuous.

[0041] With an MEA overhead of about 2%, the ECR insertion rate is at 41ECRs per second, as is so for the data stream 50 shown in FIG. 3. When,with statistical facts, a higher dynamic echo rejection rate isdefinitely needed, a double rated ECR and MEA may be inserted, as is sofor the data stream 50 shown in FIG. 3.

[0042] The forefront equalizers 32 and 72 are of any suitable design,with various suitable designs being well known in the art. An example ofan equalizer 100 suitable for use as the forefront equalizers 32 and 72is shown in FIG. 6. A received digital ATSC data stream containing theMEA signals is supplied to a FIR filter 102 and to an IIR filter 104.The ECR signals in the MEA signals from the received digital ATSC datastream are placed into a buffer 108 and supplied to a microprocessor orother suitable logic 106. The microprocessor 106, which includes amemory (not shown) which contains a suitable version of the ECR astransmitted, compares the contents of the buffer 108 to the storedversion of the ECR as transmitted, models the impulse response of thechannel, and computes a sequence of coefficients for the filters 102 and104 which implement the inverse channel characteristics. These filtercoefficients are furnished to the filters 102 and 104 to suppress suchechoes as are present, resulting in an ATSC data stream at the output ofthe filter 104 wherein the echo components are substantially reduced.

[0043] A suitable class of ECR signals for fulfilling the task ofprobing the multipath or dispersive digital communication channel isdefined by the following equation: $\begin{matrix}{{f(t)} = {{\frac{1}{2\pi}{\int_{0}^{\Omega}{\left\lbrack {{A\quad {\cos \left( {b\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {b\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}} + {\frac{1}{2\pi}{\int_{- \Omega}^{0}{\left\lbrack {{A\quad {\cos \left( {{- b}\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {{- b}\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}}}} & (1)\end{matrix}$

[0044] where, the A, b and Ω are all parameters which are altered oradjusted in order to modify or to change the characteristics of thesignal within their entire functional family. For digitalcommunications, the Ω parameter is determined to be equal to the Nyquistfrequency of the sampling rate if this functional waveform is discretelysampled when the function is to be used as an echo cancellationreference signal for digital communication. The parameters A and b arechosen to establish a signal that has high energy and is compact intime. An illustrative set of values for the parameters of Equation (1)are as follows: A=0.707×8.6×10⁻⁶ volts; b=0.9×10⁻¹² sec²/radian; andΩ=5.38×2π×10⁶ radian/sec.

[0045] Generally, the ECR signals in this class of signals have thefollowing characteristics: (1) a high signal energy level; (2) anabsolutely white spectrum; (3) a non-cyclic characteristic; (4) ashortest time duration at a given amplitude and a given required signalenergy level; (5) a time domain signal fetching length longer than thesignal itself that does not affect its spectrum whiteness property; (6)an auto-correlation function that is a single δ function, especially forecho rejection in digital communications; (7) a continuous group delayfunction within the band of interest; and (8) based on characteristics“2” and “6” the function should be and must be a real function. Thesignal defined in Equation (1) satisfies all the specifiedcharacteristics “1” through “8”.

[0046] An illustrative waveform of this signal function of time is shownin FIG. 7. The characteristics of this class of signal as a function oftime are non-cyclical, generally flat response, and a longer signal withmany more peaks than prior signals and proportionally shorter tails.

[0047]FIG. 8 shows a flat spectrum plot of the signal defined byEquation (1), which directly and indirectly implies the above mentionedcharacteristic 1, 2, 3 and 5.

[0048] The scope of my invention is set forth in the following claims.The description of the various embodiments set forth herein isillustrative of my invention and is not intended to limit the scopethereof. Variations and modifications of the embodiments disclosedherein will become apparent to those of ordinary skill in the art uponreading this document, and alternatives to and equivalents of thevarious elements of the embodiments will be known to those of ordinaryskill in the art. These and other variations and modifications of theembodiments disclosed herein may be made without departing from thescope and spirit of the invention as set forth in the following claims.

What is claimed is:
 1. An equalizer comprising: an extraction circuitfor extracting a copy of an echo cancellation reference signal from adigital data stream transmitted over a transmission path; amicroprocessor coupled to the extraction circuit for calculating filtercoefficients from the extracted copy of the echo cancellation referencesignal; and a filter receiving the digital data stream at an inputthereof and coupled to the microprocessor for receiving the filtercoefficients and substantially canceling echo interference from thedigital data stream using the filter coefficients; wherein the echocancellation reference signal is non-cyclic, has a substantially flatfrequency response within the bandwidth of the transmission path, has alarge plurality of amplitude peaks over a time interval, and hasproportionally shorter tails relative to the large plurality ofamplitude peaks.
 2. The equalizer as in claim 1, wherein the echocancellation reference signal is of a class of signals substantiallydefined by${f(t)} = {{\frac{1}{2\pi}{\int_{0}^{\Omega}{\left\lbrack {{A\quad {\cos \left( {b\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {b\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}} + {\frac{1}{2\pi}{\int_{- \Omega}^{0}{\left\lbrack {{A\quad {\cos \left( {{- b}\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {{- b}\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}}}$

A, b and Ω being real numbers.
 3. The equalizer as in claim 2, whereinA=0.707×8.6×10⁻⁶ volts; b=0.9×10⁻¹² sec²/radian; and Ω=5.38×2π×10⁶radian/sec.
 4. An apparatus for substantially eliminating echoesoccurring during transmission of a digital data stream comprising aplurality of field syncs over a transmission path, a plurality ofsegments of symbol spaces being between successive ones of the fieldsyncs, and a plurality of echo cancellation reference signals betweenthe field snycs, comprising: means for receiving the digital data streamafter transmission; means for extracting copies of the echo cancellationreference signals from the received digital data stream; means forcalculating filter coefficients from the extracted copies of the echocancellation reference signals; and means for substantially cancelingecho interference from the received digital data stream using the filtercoefficients; wherein the echo cancellation reference signals arenon-cyclic, have a substantially flat frequency response within thebandwidth of the transmission path, have a large plurality of amplitudepeaks over a time interval, and have proportionally shorter tailsrelative to the large plurality of amplitude peaks.
 5. The equalizer asin claim 4, wherein the echo cancellation reference signals are of aclass of signals substantially defined by${f(t)} = {{\frac{1}{2\pi}{\int_{0}^{\Omega}{\left\lbrack {{A\quad {\cos \left( {b\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {b\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}} + {\frac{1}{2\pi}{\int_{- \Omega}^{0}{\left\lbrack {{A\quad {\cos \left( {{- b}\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {{- b}\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}}}$

A, b and Ω being real numbers.
 6. The equalizer as in claim 5, whereinA=0.707×8.6×10⁻⁶ volts; b=0.9×10⁻¹² sec²/radian; and Ω=5.38×2π×10⁶radian/sec.
 7. The equalizer as in claim 4, wherein the extracting meanscomprises means for extracting only one echo cancellation referencesignal from between successive ones of the field syncs.
 8. The equalizeras in claim 4, wherein the extracting means comprises means forextracting a plurality of echo cancellation reference signals frombetween successive ones of the field syncs.
 9. An apparatus formodifying an ATSC VSB data stream prior to transmission to a receiver tosupport an enhanced echo cancellation capability in the receiver, theATSC VSB data stream comprising a plurality of field syncs and aplurality of segments of symbol spaces between successive ones of thefield syncs, the apparatus comprising means for inserting a plurality ofecho cancellation reference signals between the field snycs, the echocancellation reference signals being of a class of signals substantiallydefined by${f(t)} = {{\frac{1}{2\pi}{\int_{0}^{\Omega}{\left\lbrack {{A\quad {\cos \left( {b\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {b\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}} + {\frac{1}{2\pi}{\int_{- \Omega}^{0}{\left\lbrack {{A\quad {\cos \left( {{- b}\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {{- b}\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}}}$

wherein A, b and Ω are real numbers.
 10. The apparatus as in claim 9,further comprising means for inserting blank symbol spaces about theecho cancellation reference signals.
 11. The apparatus as in claim 10,wherein A=0.707×8.6×10⁻⁶ volts; b=0.9×10⁻¹² sec²/radian; andΩ=5.38×2π×10⁶ radian/sec.
 12. A method for substantially eliminatingechoes occurring during transmission of a digital data stream comprisinga plurality of field syncs over a transmission path, a plurality ofsegments of symbol spaces being between successive ones of the fieldsyncs, and a plurality of echo cancellation reference signals betweenthe field snycs, comprising: receiving the digital data stream aftertransmission; extracting copies of the echo cancellation referencesignals from the received digital data stream; calculating filtercoefficients from the extracted copies of the echo cancellationreference signals; and substantially canceling echo interference fromthe received digital data stream using the filter coefficients; whereinthe echo cancellation reference signals are non-cyclic, have asubstantially flat frequency response within the bandwidth of thetransmission path, have a large plurality of amplitude peaks over a timeinterval, and have proportionally shorter tails relative to the largeplurality of amplitude peaks.
 13. The method as in claim 12, wherein theecho cancellation reference signals are of a class of signalssubstantially defined by${f(t)} = {{\frac{1}{2\pi}{\int_{0}^{\Omega}{\left\lbrack {{A\quad {\cos \left( {b\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {b\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}} + {\frac{1}{2\pi}{\int_{- \Omega}^{0}{\left\lbrack {{A\quad {\cos \left( {{- b}\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {{- b}\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}}}$

A, b and Ω being real numbers.
 14. The method as in claim 13, whereinA=0.707×8.6×10⁻⁶ volts; b=0.9×10⁻¹² sec²/radian; and Ω=5.38×2π×10⁶radian/sec.
 15. The method as in claim 12, wherein the extracting stepcomprises extracting only one echo cancellation reference signal frombetween successive ones of the field syncs.
 16. The method as in claim12, wherein the extracting step comprises extracting a plurality of echocancellation reference signals from between successive ones of the fieldsyncs.
 17. A method for modifying an ATSC VSB data stream prior totransmission to a receiver to support an enhanced echo cancellationcapability in the receiver, the ATSC VSB data stream comprising aplurality of field syncs and a plurality of segments of symbol spacesbetween successive ones of the field syncs, the method comprisinginserting a plurality of echo cancellation reference signals between thefield snycs, the echo cancellation reference signals being of a class ofsignals substantially defined by${f(t)} = {{\frac{1}{2\pi}{\int_{0}^{\Omega}{\left\lbrack {{A\quad {\cos \left( {b\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {b\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}} + {\frac{1}{2\pi}{\int_{- \Omega}^{0}{\left\lbrack {{A\quad {\cos \left( {{- b}\quad \omega^{2}} \right)}} + {j\quad A\quad {\sin \left( {{- b}\quad \omega^{2}} \right)}}} \right\rbrack ^{{j\omega}\quad t}\quad {\omega}}}}}$

wherein A, b and Ω are real numbers.
 18. The method as in claim 17,wherein the inserting step comprises inserting only one echocancellation reference signals between successive ones of the fieldsnycs.
 19. The method as in claim 17, wherein the inserting stepcomprises inserting a plurality of echo cancellation reference signalsbetween successive ones of the field snycs.
 20. The method as in claim17, further comprising inserting blank symbol spaces about the echocancellation reference signals.